1. Field of the Invention
The present invention relates to an oscillation controlling apparatus, a recording medium having a program recorded thereon, and a channel selecting apparatus.
2. Description of the Related Art
In FM radio receivers, AM radio receivers, etc., oscillation circuits are used to extract signals of desired broadcast stations from received signals and to convert received signals into intermediate frequency signals. Such an oscillation circuit includes a coil, a capacitor, and a varicap (variable-capacitance diode), for example. Capacitances of the capacitor and varicap are changed in accordance with a value of a control signal input from a microcomputer, etc., and the oscillation frequency of the oscillation circuit is changed into a target frequency to extract a signal of a desired broadcast station and to convert a signal into an intermediate frequency signal (e.g., Japanese Patent Application Laid-Open Publication No. 2002-111527).
In the oscillation circuit changing capacitances of the capacitor, varicap, etc., to adjust an oscillation frequency, due to effects of temperature characteristics and manufacturing variances, the value of the control signal cannot be set in advance to set the oscillation frequency to the target frequency. Therefore, the value of the control signal must be obtained to set the oscillation frequency to the target frequency at the timing of changing the oscillation frequency.
In one method of obtaining the value of the control signal corresponding to the target frequency, the value of the control signal is changed stepwise within a variable range to obtain the value of the control signal setting the oscillation frequency to the target frequency.
However, since the value of the control signal is changed stepwise in this method, it takes very long time to obtain the value of the control signal corresponding to the target frequency.
In another method of obtaining the value of the control signal corresponding to the target frequency, linear approximation is used. In the method using linear approximation, an approximate line showing the frequency characteristic of the oscillation frequency is obtained based on oscillation frequencies of two appropriate values of the control signal. An approximate value of the control signal corresponding to the target frequency is obtained in accordance with this approximate line. The value of the control signal is then changed near the approximate value to obtain the control signal corresponding to the target frequency.
However, the characteristic of the oscillation frequency in the oscillation circuit is a curve such as a quadratic curve, and the approximate value of the control signal obtained from the approximate line may differ greatly from the value of the control signal corresponding to the target frequency. Therefore, a large range must be defined for changing the control signal near the approximate value of the control signal, and it takes long time to obtain the value of the control signal corresponding to the target frequency.